1. Field of the Invention
This invention relates to a process for controlling the average molecular weight and the molecular weight distribution of .alpha.-olefin polymers and copolymers. This invention is directed to a process for preparing olefin polymers of average molecular weight of 50,000 to 1,000,000 and S values of 7 to about 13.
2. DISCUSSION OF THE PRIOR ART
According to the process developed by Ziegler, catalysts consisting of compounds of the transition metals of groups 4 to 6 of the Periodic Table of elements, preferably titanium compounds, and of organometallic compounds of elements of groups 1 to 3 of the Periodic Table, especially aluminum alkyls or alkyl aluminum halides, are used for the polymerization and copolymerization of alpha olefins at low pressure. The reaction of the olefins to form polymers is preferably carried out in suspersion or solution but may also be carried out in the gas phase.
The process described above gives products which can be processed to injection molded articles, hollow articles, pipe, tube or film by injection molding, blow molding and extrusion processes. Each molding process and each field of use requires products having different physical properties for which especially the average molecular weight of the polymer and the distribution of molecular weights are decisive. Both of these factors take account of the fact that the macromolecular materials obtained by the synthesis are invariably polymolecular materials. They consist of macromolecules which are built up from the same basic units but differ by the degree of polymerization. The statement of the average molecular weight of a macromolecular material represents, therefore, merely the average value of the molecular weight for the particular polymolecular mixture.
Various methods such as osmometry, measurement of scattering of light, viscosimetry and sedimentation behavior in the ultracentrifuge are available for determining the average molecular weight.
However, many properties of macromolecular material such as toughness, hardness, elasticity, solubility or processibility, e.g., by extrusion, are not only determined by the average molecular weight but depend also on the scattering or dispersion of the molecular weight of the macromolecules of various size present as mixture. High impact strength (notched) which is a criterion of brittleness and toughness of a material is characteristic of polymers having narrow molecular weight distribution. Polymers having a broad molecular weight distribution are characterized by improved flow and enhanced resistance to stress cracking corrosion. Therefore, it is not sufficient to describe a macromolecular material only by the average molecular weight, it is rather necessary to supplement this value by mentioning the molecular weight distribution.
To measure its molecular weight distribution, the macromolecular material is separated into individual fractions and the quantity and molecular weight of these fractions are determined. This method is time-consuming and circumstantial. Therefore, one is contented in general with an approximative determination of the molecular weight distribution. One possible method is the determination of the flow behavior of the polymers. For example, the quotient from the melt indices of a material measured at different loads (Melt Flow Index.sub.5 or Melt Flow Index.sub.15 according to Deutsche Industrie Norm 53 735) is a measure of the width of its molecular weight distribution. The quotient from MFI.sub.5 and MFI.sub.15 is referred to as S value. Low S values mean narrow and high S values mean broad molecular weight distributions. For example, S values of about 5 to about 20 are characteristic of polyethylene.
Polyolefins having narrow molecular weight distribution, i.e., S values of 6 to 7 and low molecular weight of about 20,000 to 40,000, are particularly useful for processing by injection molding while products having a broad molecular weight distribution, i.e., S values of 13 to 17, and relatively high molecular weights of about 80,000 to 200,000 can be processed particularly well by extrusion.
Control of a molecular weight which is favorable for further processing is effected by varying the reaction conditions, especially the polymerization temperature, varying the ratio of the catalyst components or adding chain transfer agents to the reaction mixture. Hydrogen is preferred as chain transfer agent. Corresponding processes are disclosed, for example, in German Pat. No. 1,420,390 and German Auslegeschrift (DE-AS) 1,595,666. The processes described in the publications mentioned above give products which have a narrow molecular weight distribution, i.e., S values of 6 to 7, and are preferably suitable for processing by injection molding. In the process according to German (BRD) Offenlegungsschrift No. (DE-OS) 1,720,611, ethylene or mixtures of ethylene with up to 10% by weight of higher alpha olefins are polymerized in two steps in suspension or in the gas phase while controlling the average molecular weight with hydrogen to achieve a broader molecular weight distribution. Monomer mixtures of different compositions are preferably used in the two steps.
Apart from hydrogen, alcohols and/or oxygen are also used in some cases for controlling the average molecular weight. Such a method in which catalysts of titanium tetrachloride and dialkyl aluminum monochloride are used is described in German Pat. No. 1,210,987. It gives polymers having S values of about 13 to 15.
The processes described above for the control of the average molecular weight and the molecular weight distribution do not meet all requirements of practice. In particular, they do not provide a simple route to the production of polymers which are useful for the production by extrusion of molded articles having smooth surface and high toughness. While it is possible by addition of hydrogen in the polymerization of alpha olefins to control the average molecular weight within wide limits, the resultant products show a narrow molecular weight distribution with the disadvantages mentioned above. On the other hand, addition of alcohols and/or oxygen permits the production of polymers having a broad molecular weight distribution. With this control system, the average molecular weight can be varied only within narrow limits. Multistage processes for the production of polymers having specific average molecular weights and specific molecular weight distributions are technically expensive and circumstantial and can be carried out only discontinuously in many cases and, therefore, are not always suitable for the economic production of the polymers.
A method solving the problems mentioned above is described in German (BRD) Offenlegungsschrift (DR-OS) No. 2,630,262 which describes a process for the polymerization or copolymerization of ethylene with the use of catalysts which contain a trialkyl aluminum compound as one constituent, wherein oxygen in addition to hydrogen is added to the olefin or olefin mixture. This process gives polymers which have the properties desired, but the relatively high consumption of catalyst appears to be disturbing in large-scale operation of the process.